Directionally sensitive shallow ionization chambers for neutron well logging



July 28, 19 59 DIRECTIONALLY SENSITIVE SHALLOW IONiZATION CHAMBERS FORNEUTRON WELL LOGGING Original Filed July 30, 1949 R. E. FEARoN ETAL2,897,372

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INVENTORS ROBERT E. FEARON y JEAN M. THAYER ATTORNEY July 28, 1959CHAMBERS FOR NEUTRON WE'LL. LOGGING 4 Sheets-Sheet 2 Original Filed July30 TO SURFACE w m 0E M F Y N.H W EE T s W M N IR E EN T BA/ N 3 O E H RJ0 N w T m w m A .lllllul-lll" IFQMO 5 MN E m H C RT 7 6 5 4 & M l I l Il 1 I I I.|| W 3 3 3 3 f 9 6 SN J j 2 2 7 7 7 m E 4 a c \7 L 5 2 mH R NMR L 3, v c 2 A H U P mo P DT M CT. M A2 A A m A A A: o T M. c IN 1 m 5s DI \I| I S III n B A n i FT o f n 0 0 d t d A 9 j F l I l lhlllfllllLATTORNEY July 28, 1959 DIRECTIONALL'Y CHAMB SHAFER PULSE HEIGHT SELECTORNEUTRON SOURCE E. FEARON EI'AL SENSITIVE SHALLOW IDNIZATION ERS FORNEUTRON WELL LOGGING Original Filed July so, 1949 I 4 snZets-"sneet aOUTPUT JNVENTORS ROBERT E. FEARON JEAN M. THAYER ATTORNEY y 1959 R. E.FEARON ET AL 2,

DIRECTIONALLY SENSITIVE SHALL-OW IONIZATION CHAMBERS FOR NEUTRONWELL-LOGGING 1949 4 Sheets-Sheet 4 Original Filed .July 30 INVENTORSROBERT E. FEARON JEAN M. THAYER ATTORNEY S irs DmEcTIoNALLY SENSITIVE,sHALLow ,IONI'ZAY- TIQN CHAMBERS non NEUTRON WELL LOG- GING Robert E.Fearon and JeanM. Thayer, Tulsa, Okla assignors to Well Surveys, Inc, acorporation of Delaware 11 Claims. (Cl. 250-83.6)

This invention relates generally to a method and apparatus foridentifying substances existing in difiicultly accessible locations, forexample, adjacent to a deep narrow drill hole, and more particularly toa method and apparatus for identifying and distinguishing thesesubstances from each other by nuclear reactions in the substances. Thisis a division of our copending application, Serial No. 107,806, filedJuly 30, 1949, now Patent No. 2,712,081 for a Method and Apparatus forNeutron Well Logging.

This invention is directed to the solution of a problem which has beenlong recognized by geologists and geophysicists, and by others,confronted with the problem of locating valuable substances, such aspetroleum, in the sub-surface formations of the earth. The problem ofdiscovering with certainty the existence of a particularly valuablesubstance in the sub-surface formations of the earth has only beenpartially solved by the prior art workers. All prior efforts to solvethe problem have met with failure for the reason that no parameter couldbe found which was solely characteristic of the valuable substances thatit was desired to locate. As an example, in the art of well logging apartial solution to the problem goes as far as determining withcertainty that either salt water or petroleum exists in a particularformation but a complete solution is not possible, since prior to thisinvention, no parameter was known whereby the two substances could bedistinguished, in situ, from each other.

Numerous other methods advanced by the workers in the prior art forlocating valuable substances in the subsurface formations of the earthinclude: electrical methods which involve the measurement ofself-potential, conductivity, and resistivity; thermal methods; seismicmethods which treat of the acoustical properties of the subsurfaceformations; natural radioactivity of the formations; and those methodsin which the formations are irradiated with radioactive radiations andan effect such as the gamma radiation produced by the neutroninteractions in the formations measured. All of these methods as well asothers which have not been enumerated above, have not afforded acomplete solution to the above problem in that none of them measures aparameter that is solely characteristic of the valuable substances thatone is desirous of locating. v s

For the purpose of particularly describing and setting forth therespects in which this invention differs from and represents advancementupon the prior art, there is set forth a description of the efforts ofprevious workers insofar as they have been directed to the problem whichhas been stated in the previous paragraph.

The location of petroleum has been attempted by various well loggingmethods which are sensitive to some physical characteristic imparted tothe rocks by the presence of petroleum in them. For example, resistivitymethods in combination with other methods somewhat ambiguously enabledetection of petroleum. The incontent venienee and, uncertainty of theuse of resistivity methods arise from the fact that resistivity is ageneral property of rocks, and is possessed by some rocks not Containingpetroleum to an even greater. extent than the degree to which theproperty is manifestedbyv certain other rocks full of petroleum. Forexample, Indianalimestone will be found to have a much higherresistivity than on saturated sandstone of the Frio formation in thegulfcoast oil fields. Furthermore, sandstone which contains natural gas, hasa high resistivity, as does also coal. Moreover, limestone may show adecrease of resistivity where an oil bearing horizon appears. -It couldsimilarly be shown how each and every one of the other non-nuclearlogging methods have specific shortcomings which analogously preventthem from being or amounting to a specific recognition of petroleum.

The instant invention provides a complete solution to the above problem.This solution consists of a system of observations by which the operatoris enabled to recognize and quantitatively measure nuclear species ofthe subsurface formations adjacent a bore hole. Although the desiredsubstances quite often are not elements or single nuclear species thechemical laws of combining proportions enable accurate appraisal of suchthings as the occurrence of petroleum. Recognition of nuclear species isaccomplished by subjecting the substance adjacent to the bore hole tobombardment with penetrating radiations of a nature to cause specificand determinative quantized changes in the potential energy of the saidnuclear species. These quantized energy changes which are specific tothe particular kinds of atoms to be determined are used as a means ofrecognizing the desired atoms, which recognition is accomplished bymeans of selective neutron detection, selective for specific energyranges of neutrons, and/ or specific limits of direction of incidenceand sense of direction of incidence;

Among the means which are required for the solution of the aboveproblem, there is provided exceedingly powerful and energeticallyefficient monoenergetic neutron sources, relying upon the nuclearreactions caused by electrical or electromagnetically acceleratedparticles. These are provided in a form which is' adapted to be loweredinto a bore holegand employed therein to bornbard the rocks adjacent tothe bore hole. Also required for the practice of this invention arepowerful capsuled neutron emitt'ing sources, depending for theiroperation upon energetic particles emitted by radioactive substances.There is set forth the manner of choosing and designing suchneutron-emitting source's, showing how a person skilled in the art canavail himself of intensities hundreds of times greater thanthosewhi'chare now available.

Required in the practice of this method are various means of observingneutrons which permit the determination of the energy, the direction ofincidence of neutrons, and the sense of direction.

Among these means, there are provided devices which determine bothenergy and direction of incidentv neutrons within certain limits. Thereis also provided a device for detecting phenomena described in nuclearphysics as n-p reactions. This device enables exact determination ofenergy of neutrons, and a somewhat ambiguous determination of direction.Incidental to the practice of this invention also is a device forresolving nuclear data which gives only a general indication of energy,and interpreting this general indication of the energy of neutrons in amore exact way. There is also provided, as a means of practicing thisinvention, a choice of the manner of employment of a number of neutronfilters adapted to select specific energy groups of neutrons. It isshown liow these filters may be. employed for the purpose of identifyingspecific elements in the strata.

Therefore the primary object of this invention is the provision of amethod and apparatus for identifying valuable substances by, separatelymeasuring the influence of specific properties of the nuclei of thevaluable substances upon a flux of fast neutrons.

Another important object of this invention is the provision of a methodand apparatus whereby petroleum can be positively identified in thesubsurface strata adjacent a bore hole.

This invention also contemplates a method and means for locatingvaluable substances situated in diliicultly accessible locations byidentifying and measuring the influence of at least one of itselementary components on a flux of fast neutrons.

Still another object of this invention is to achieve the above objectsby irradiating formations with fast neutrons and measuring the intensityof neutrons falling within specific energy bands and which haverebounded from the formations.

Another object of this invention is to provide a method and means forproducing a log of a drill hole by recording versus depth the averagerate of occurrence of processes occasioned by fast neutrons of selectedenergies which enter the detecting device.

A further object of this invention resides in the provision of a methodand means for detecting neutrons, selecting pulses produced therebywhose energies lie in a predetermined range, and recording theirtime-rate of occurrence versus depth.

Another object is to provide means for delivering to a recorderelectrical signals which denote the intensity of neutrons of a definiteenergy class.

This invention also contemplates means for determining specific energylosses in samples of substances exposed to neutrons of a determinedenergy for the purpose of adjusting energy selective neutron detectorsystems used in well logging.

Another object is to provide a method and means to accomplish deepinvestigation in a direction perpendic ular to a bore hole andconcurrently provide very detailed resolution of thin strata.

Still another object is to provide detectors which are directionallysensitive and which are adjusted with respect to the source forfavorable angle of fast neutron scattering from formation substance thatare capable of determining variations of the properties of strata withdistance horizontally.

This invention also contemplates a novel detector whereby dipdeterminations can be made in a drill hole. Another object of thisinvention is to provide a detectmg system whereby horizontal anisotropycan be detected.

Still another object is to provide means for detecting horizontalanisotropy in a measurement based upon a particular energy of neutrons.

A further object is to provide a detecting system whereby verticaldirection sensitivity and sense of direction of neutrons of a particularenergy can be detected.

Other objects and advantages of the present invention will becomeapparent from the following detailed description when considered withthe drawings, in which Figure l is a schematic illustration of a welllogging operation showing the surface recording system;

I Figure 2 is a diagrammatic illustration of a subsurface instrumentwith the detector illustrated in vertical section;

Figure 3 illustrates the type of well log that would be produced by thepresent invention.

Figure 4 is adiagrammatic illustration of a well logging systememploying a novel type of detector, and a novel detector-sourcearrangement;

Figure 5 is a detailed wiring diagram illustrating the contents of oneof the elements utilized in Figure 4;

Figure 6 illustrates a modification of the detector shown. in Figure 4;

Figure 7 illustrates still another modification of the detector shown inFigure 4;

Figure 8 is a cross-sectional view of the detector shown in Figure 7taken along theline 8-8 as indicated;

Figure 9 illustrates still another modification of the detector shown inFigure 4.

As pointed out above, consideration of the problem of well logging hasled to the conclusion that there is a necessity for the discovery ofmethods which will identify more specifically the substances found inthe rocks adjacent to wells which are logged. Specific identifyingproperties, which could be relied upon as a means of recognition ofsubstances, must be able to cause an efiect which is observable underthe logging conditions which prevail. Preferably the process making theobservations possible should be one which acts through space and throughmatter which fills the space between the position in which the rock tobe identified is found, and the location of the detecting apparatus inthe bore hole. The necessity for action through space arises because ofthe prevalence of casing and/ or cement and/0r fluid of various sortswhich commonly exist in the well bores, and which interfere with themeasuring process. Another reason why considerable action through spaceis esscn tial is the need for the depth of investigation to he adequate.Considerable depth of investigation is a highly desirable factor in welllogging because of the heterogeneity of rocks making shallowobservations unrepresentative, and therefore inaccurate as arepresentation of the whole mass of rock penetrated.

There are available at the present time only a very few types ofinfluences by which desirable observations as discussed above may bemade. Obviously, the magnetic and electric fluxes are not available forconsideration in connection with cased wells, and the electric flux isunusable when investigating non-conducting material. The observation ofthe heat flux is familiar in the art of well logging and has patentlythe disadvantage that such observations are slow if one desires aconsiderable depth of investigation. The transmission of observableinfrared and ultraviolet radiations is excluded because of the opacityof substances generally present in the earth and in bore holes. Thegravitational flux has satisfactory properties, and, in principle, couldbe measured. But no known means of measuring it for Well loggingpurposes has been found.

In attacking the above problem, seeking for a method of specificrecognition of material in the circumstances of a bore hole penetratingthe rock strata of the earth. it has been discovered that there areapparent specific properties of atomic nuclei corresponding with energytransitions in those nuclei. These transitions may evidence themselvesin a variety of ways, such as:

(a) The emission of radiant energy through space.

(12) The absorption of a particular amount of energy from a bombardingparticle or quantum.

(c) A specific energy threshold or a plurality of energy thresholds ofsusceptibility of the nuclei to certain classes of nuclear change, whichmay be caused by bombarding corpuscles or quanta.

It has been discovered that in all branches of molecular, atomic, andsub-atomic physics, one may generally predict that if a specific energytransition is possible in a quantized system, there will be a resonanceeffect, specifically affecting bombarding particles or quanta possessingenergy (either kinetic or potential) in the close vicinity of the amountrequired to produce a quantized transition. The discovery of the detailsof quantization of nuclei of atoms waits still for extensiveexperimental and theoretical work. Limited experimental evidence hasalready brought support to the conviction which exists in the minds ofall nuclear physicists to the effect that nuclei will surely be found tobe quantized systems. In some instances energy thresholds of variouskinds have already been determined for nuclei. For example, the

a ses-7' photomeutron thresnurcl is now known experimentally throughpthestudy of .its inverse process, .capture, by Kubischek and Dancoff. l I p7 -A specific energythreshold at 20 megavolts has been found for thesystem comprising 4 nucleons protons and 2. neutrons Sundry isomerictransitions corresponding with :highly forbidden transformations of thearrangements of nucleons have been found experimentally and can beconsidered as additional evidenceof the truth-and experimentalsignificance of the general conclusion that nuclear matter exists inquantized energy states.

7 In an effort to make use of the foregoing general conclusion, :it hasbeen :discovered that only two classes of radiation appear to: existwhich react with nuclear matter appreciably and can be arrangedconvenientlyfor the observation of quantized energy levels-of nuclei.These classes of radiation are the photon or electromagnetic class, andthe corpuscular class comprising neutrons. Other particles (charged) ingeneraldo not penetrate the coulomb field of force surrounding a nucleusat energy falling in the range of possible excitation processes ofcommon nuclei. Such excitation processesare typically expectedv forlight nuclei in the vicinity'of l-millionelectron volts. Chargedparticles lack action through a distance as defined herein. Tlherefore,corpuscular radiations of the charged variety would, in principle, notbe particularly useful for investigation -of the quantized levels ofnuclei. Of the classes .of radiation which have beensuggested, the onlyone which hasbeendiscovered which has a favorable ratio for theamount'of interaction which it undergoes with nuclear matter, ascompared-with the energy transitions effected in the progress of theradiation by circumstances arising outside the-nuclei of 'atoms, is theneutron. The photon .reacts extensively with orbitalelectrons, and hasonly a very small cross'section '(target probability) for interactionswith nuclei as such. There is furthermore an additional reaction ofphotons which becomes prominent above 2 electron megavolts, and which,=in-the range above 2 electron megavolts results'in mat'erialization ofelectron-positron pairs. This 'mate-rialization, though influenced bythe; presence in-the near vicinity of the nuclear field of force, doesnot represent a specific or identifyingcharacteristic of particularnuclei, but is a general characteristic-of all nuclei, more prominentfor'th'e nuclei 'ofheavy elements likelead and lessprominent'for the*nuclei of light elements such as aluminum. For the above listedreasons, there appear to be only a few especially simple reactionscaused by photons whichrmight be of any use. One might find it-desirableto observe the neutrons released from nuclei -by photons, since thereis,for such' nuclear photo-neutrons, a specific threshold of energy foreach'nuelear species (element or'isotop'ethereof). One might alsoinvestigate 'the unmodified Compton scattering of energetic photonradiations in 'thehope of finding some slightly modified lines whichsuffered loss of energy byinteraction with nuclei. This possibility issomewhat favored by the fact that the otherwise much stronger modifiedCompton scattering radiation is rapidly eliminated from the flux byabsorption.

On the other hand, the interaction of neutrons with the outside parts ofthe atom is so small that the direct production of ion-pairs by neutronsis found to occur on an average of only about one time per meter ofordinary atmospheric air for a neutron possessing a kinetic energy offive million electron volts. The liberation of energy by neutrons in airtherefore amounts to less than onethousandth of 1% per meter of airtraversed, for energy liberated by processes involving the'outsideportions of the atoms found in the A distance of travel in air whichwould result in 'an'average loss of energy by reaction with outsideparts of the atoms of less than 11%,, would,.never- 'theless, resultinjtotal absorption'of theneuttons, and all 5 their energy, by reactionwiththenuclei of the atoms contained in air. Even any dffthere actionswhich neutrons undergo, which occurfbetweenneutrons and nuclei of thematter,ar'e'not'higlily specific, nor do they in any refined efforts .toidentify matter. 7 Among the unidentifying nuclear reactions one mayname, {or ex ample, conservative ballistic scattering of .neutrons,that'is, conservative of total kinetic energy. This process isspecifically different to an extreme degree only in the case of verylightfelernents suchas hydrogen and helium. The average nature of othermattercontained in the rocks is sufiicient'ly alike inthis respectthatthemain possibility of use of theproperty ofconservative'ballisticnuclear scattering of .Ineutrons lis 'to observe differences in thepropagation of neutrons through therock which enable conclusionsregarding the presence ofhydrogen to be made. This effect is alreadymade nse of, and there exist a considerable number of US. patents andother published descriptions'bearing on this subject. Amongth esepatents are No. 2,308,361, .No.. 2,2ZQ,-5.09, and No. 2,349;r 712. Thebroad class under which these previously named inventionsfa'llcorrespondswith apatent-issued to John C. Bender, No. 2,133,776. v

The theory of detection of hydrogen by conservative ballistic nuclearscattering is treated in .an article written by Robert E. 'Fearon andpublished in the June 1949:, issue of Nucleon'ics, entitled NeutronWelLLogging. The above theory findsgeneral application in pursuing thismethod, and Figures 1 and 2'more-particularly set forth the details ofarrangements through which these general concepts find specificapplication to the problem :set forth above. 7 7 Referring to thesefigures there is illustrated an application of this invention to a wellsurveying system. 111 Figure '1 there is shown schematically a drillhole "10 which may or may not be cased. Disposedfin .the drill hole andadapted to be raised or lowered therein is a housing'll supported by acable 12. Cable 12 It comprises at "least one electricalconductonconnecting theelectrical apparatus within the housing 11 toapparatus located adjacent the mouth of the drill hole 10. The apparatuson the surface of-the earth consistsof a measuring wheel '13 over whichthe cable 12 passesand a winch .or drum 14 on which the cable iswound-,-:0r from which it is unwound, whenthe housing 11 is raised orlowered inthe drill hole 10. Conductors are connected to the cable 12 bymeans-of slip rings 15 and brushes 16 carried on one end of drum .14.These conductors lead-to an amplifier :17. Amplifier 17 is aconventional audio amplifier having a flat frequency response. Theoutput of amplifier I' 7 is conducted to a pulse shaper 18, the-purposeof which is to insure the delivery of square topped-waves of constantheight .to an integrator 19. Integrator 19 is adapted to receive theaforementioned pulses andfgenerate therefrom an electromotive forcewhich is proportional to the average time-rate ofoccurrence of the;pulses. This signal is 'delivered to the recorder 20 where it isrecorded versus depth. The-depth axis ofthe recorder is actuated by theshaft 21 which leads from a gearbox 2 2, connecting through shaft 23 tothe measuring wheel 13. The gear box '22 has adjustments to enablesuitable choice of depth scales. p I

Referring specifically to Figure 2v a description of the contents ofhousing 11 will follow. It is to be understood thathousing 11 will beconstructed to withstand thepressures, and mechanical and thermal abusesencountered in surveying a deep borehole and yet provide adequate spacewithin it to house the necessaryapparatus and-permit the transmission ofradiation throughit.

In the bottom portion of housing 11 there is located a radiation source2d which-maybe surrounded by a radiation filtering material 2 5. Thisradiation source may take various forms which will be described indetail later in the specification. Above the filtering material 25 andlying between the source" of radiation and a radiation re gae detector 26, thereis a region of space which may be occupied by suitable materialsor left vacant determined by considerations explained as the descriptionprogresses.

The detector 26 is of the type which will detect neutrons as a result ofthe production of prominent bursts of ionization therein, caused byrapid movements of heavy charged particles such as protons, alphaparticles, etc., set in motion by the neutrons. The bursts of ionizationare very quickly collected in the detector 26. These bursts areregistered as electrical pulses and resolved timewise from other orsmaller pulses which may occur almost concurrently. The detector 26 isso designed and so operated that the magnitude of the electrical pulsereleased from the collection of a specified amount of electrical chargewill always be quite accurately proportional to the amount of theelectrical charge collected and substantially independent of the path inthe detector along which the burst of ionization occurred. The currentcorresponding to a pulse, flowing in the electrode circuit whichincludes conductor 27, resistance 28, battery 29 and conductor 30,produces a voltage pulse across the resistance that is of the formillustrated at a. The pulse produced across the resistance 28 isimpressed through the condenser 31 upon the input of an amplifier 32. Asshown at b the pulse has suffered negligible loss and no distortion inpassing through the condenser 31. Theamplified pulse, illustrated at c,has been inverted in polarity but otherwise faithfully reproduced. It isthen conducted to the pulse height distribution analyzer 33. Here onlythose pulses whose magnitude fall within a prescribed range, such asillustrated at d and designated by e, are accepted and transmitted.Other pulses such asare illustrated at f and g'are not accepted andtransmitted. Those pulses which are accepted and transmitted aredelivered to an amplifier 34. Amplifier 34 is one having a flatfrequency response extending upward to the highest frequency required tofaithfully amplify the pulse delivered to it in a manner shown at h. Theoutput signal from the amplifier 34 is fed into a scaling circuit 35,which, in a known manner, delivers pulses as illustrated at i, thenumber of which occurring in a given time is less by a constant factorthan the number received in the same interval of time. The output of thescaling circuit is fed into a shaper 36 which transforms the pulse intothe shapeillustrated at i. The shaper 36 may take the form of a powderediron core transformer. The signal from the transformer is then fed intoimpedance matching means 37, such as a cathode follower, whichfaithfully reproduces the voltage wave as illustrated at k. Theimpedance matching means 37 introduces the signal into the transmissionline contained within the cable 12 for the purpose of transmitting it tothe surface.

It is to be understood that all elements within the housing 11 whichrequire power may be powered in a conventional manner as taught in theart by means such as batteries or rectified alternating current.Batteries which very satisfactorily fulfill the temperature requirementsin hot wells are the zinc, potassium hydroxide, mercuric oxide cells.

Again referring to Figure 1, the signals transmitted to the surface bymeans of cable 12 are taken therefrom by means of slip rings 15 andbrushes 16 and are conducted to the amplifier as pulses, one of which isillustrated at I. These amplified pulses are received by a pulse shaper18 which modifies their form in the manner illustrated at o. The pulseillustrated at will always have a fixed square form with a fixed heightm and fixed width n. These substantially square pulses are then fed intothe integrating circuit which delivers the signal to the recorder 21 ashas been previously described. The integrating circuit thus producesattire-dependent voltage wave such as shown'at p. When'this signal isimpressed on the recorder, which has been "coordinated with depth, acurve will be drawn as shown in Figure 3. This curve has as its ordinatedepth in the bore hole and as its abscissa a function of an intensityofreceived radiation, or of a plurality, or combination of intensities.These intensities may be combined by adding, subtracting, or dividing inany desired manner, or may be otherwise mathematically combined. v .Themanner of combination is suitable to specifically indicate, or beespecially sensitive to, the presence of a particular substance in theregion adjacent the bore hole.

Although no power supply has been shown in connection with the surfaceapparatus, it is to be understood that it will be powered in aconventional manner such as was pointed out in connection with thesubsurface apparatus.

As can be understood from previous parts of this application, it is anobject of this invention to measure only certain parts of an otherwiseless informative flux of scattered, dilfused, or partially absorbed fluxof neutrons, and to use data concerning the intensity of these dissectedportions of the neutron flux as a means of obtaining more specificinformation regarding the nature of the substance by which the primaryneutron flux is diffused, scattered or absorbed. Quite naturally,therefore, it may be seen that the measurement proposed herein is moredifiicult in certain particulars than those called for by thediscoveries of the prior art. For example, the requirement that therebe, within the interval of time'in which a measurement is performed, astatistically sufficientnumber of processes to produce an accurateobservation of the average rate of occurrence of such processes, will beless satisfactorily met. This conclusion is derived from the propositionthat this discovery concerns itself in each instance with a measurementof only a part of the neutron radiation. Probable error in themeasurement of any radiation is decreased when adequate intensityprevails, the percentage probable error in general being inverselyproportional to the square root of the intensity. For illustration,therefore, if there are neutrons composing an energy spectrum uniformlydistributed from zero to five million electron volts, and it is desiredto observe that portion of the energy spectrum lying between threethousand four hundred electron kilovolts, and three thousand fivehundred electron kilovolts, the percentage probable error of such ameasurement will be approximately seven times worse than it would be ifthe measurement had used all the neutrons. It follows, therefore, thatstrong fluxes of neutrons are needed to practice this well loggingmethod. It likewise follows that, if the neutrons are to be undirected,there is need that they be generated in some isotropic nuclear process.

Another form of the instant invention is illustrated diagrammatically inFigure 4. This form of the invention is adapted to measure hydrogencontent of the formations adjacent a drill hole. The measurement iseffected by detecting maximum energy neutrons entering a specialdetector substantially perpendicularly to the axis of the instrumentafter they have been emitted by a source vertically spaced 3. shortdistance from the detecto'r, entered the formations, and rebounded fromrelatively massive nuclei therein. These energetic processes which aremeasured are those which correspond with neutrons entering the specialdetector, making a substantially square hit on a helium (or hydrogen)nucleus in the detector, and causing the recoiling nucleus to be sodirected in the detector, that it dissipates its entire energy in theionizable medium therein, along a path transverse to the axis of: theinstrument. Neutrons, which are detected are, for square hits, collinearwith the recoil paths they cause. Therefore, the special detector, asused herein, is quite accurately directional and will be sensitive onlyto neutrons of maximum energy, incident perpendicularly to the axis ofthe instrument. l

It has been made certain that this directional sensitivity will prevailby selecting the shape of the gas space in the detector between itswalls so that long recoil .paths can only exist perpendicular to' theaxis.

Neutrons which have maximum energy cannot have rebounded but very fewtimes from atoms in the strata, because those which have rebounded moretimes will .generally not have retained maximum energy. Forsimplicity,consider a maximum energy neutronwhich has left the sourcesubstantially .perpendicularly'to the axis of the instrument, and hasrebounded in a suitable direction to enter the detector. .Such a neutroncan retain approximately its total energy, only if the atom it struck inrebounding was not hydrogen. Neutrons striking hydrogen nuclei under theabove circumstances will have lost nearly all of their energy. Hydrogenwill, therefore, be very specifically measured because it will produceadeficiency of returned energetic neutrons, compared with the intensitywhich is returned from similar strata not containing hydrogen.

Referring particularly to Figure 4 for a description of the apparatus,there is shown a housing 137 that has been designed for operation ina-drill'hole'in which high pressures are encountered. Housing 137contains, in its lower part, a source of neutrons 138, a battery 139 anda variable connection thereto indicated as 140. Battery 139 serves toplace a potential on the electrodes '141 and 142 of the impulseionization chamber 143. Electrodes 141 and 142 are disposed in anatmosphere of' helium or hydrogen. These electrodes are in the form ofdiscs which are axially spaced, from each other an amount which is manytimes less than their diameter.

-In a practical well logging instrument the spacing between electrodescan be 1 cm. for a diameter of electrodes of 7cm. and a helium pressureof approximately 7 atmospheres, or a hydrogen pressure of approximately'100 atmospheres.

In order to cause equal weighting of the influence upon the externalcircuit for electrons produced in the ionization chamber, there has beenprovided a screen .144. The screen 144 is connected to the variable tap140 of the battery 139. The screen is so chosen that it -is apractically perfect electrostatic shield for the electrode 141. This isaccomplished, as is well known, by

using screen having a spacing between its wires which is small comparedto the distance between the screen 144 and the electrode 141. Thisarrangement results in equal weight being given all electrons whichoriginate in the major part of the interior volume, that is, betweenscreen 144 and electrode 142.

Except for screen 144, the electrode circuit includes electrodes 141 and142, battery 139, the ground or casing 137, and resistor 145. Processesoccurring within the ionization chamber 143 cause to be produced, acrossthe resistor 145, pulses which may be amplified by the amplifier 146.Amplifier 146 is preferably one having 'a flat frequency response in therange of from 100 kilocycles to 10 megacycles. The amplified signals, orpulses, are fed into a pulse height selector 147. The output'of thiscircuit is in turn conducted to a pulse shaper and impedance matchingcircuit, indicated by block diagram at 148. The impedance matching meansare necessary in'delivering to the transmission cable signals ofsuitable nature to be transmitted to the recording system located on thesurface of the earth adjacent the mouth of the drill hole. It is to beunderstood that the recording system will have a pulse rate conversioncircuit, such as has been described heretofore, connected therein.

The pulse height discriminator '147 may take the form illustrateddiagrammatically in Figure 5. In this figure there is shown inputconductors 149 which serve to conduct a signal to the grid of a vacuumtube 150, which is normally biased beyond cutoif, by an amount sufii-Jcient to reject all but'the largest incoming pulse signals. Tube 150has its output connected :by. means of a cathode ,follower to thesucceeding'circuit element 148.

Since the method and apparatus just described is a directed ray methodof investigation, it is exceptionally well adapted to make deepinvestigations of very thin strata, without error being caused by "thenear presence of other and different strata above and below.

A modification of that form of the invention described immediately aboveis shown in Figure 6. In this modification the ionization chamber 143 isshown having the planes of its electrodes disposed at a substantiallyacute angle to the axis of the instrument. This angle may be any angledesired which will align the space between the .electrodes with the pathof rebounding fast neutrons which it is desired to measure. In thisfigure, corresponding elements have the same reference characters. Theelectrical output circuit extending to the transmission cable may be thesame as that utilized in connection with Figure 4.

The difference between the arrangement of Figure 4 and the arrangementof Figure 6 is that, by means of the Figure 6 arrangement it is possibleto direct attention to the most favorable angles of scattering, at whichthe primary maximum energy neutrons of the source have the largestlikelihood of being scattered by the nuclei of atoms present in thestrata. Furthermore, since this favorable angle is realized only in theplane including the axis and the lowest part of the tilted ionizationchamber, and only at the left of the axis in this plane, as shown in thefigure, the arrangement of Figure 6 will be orientation sensitive in abore hole, and may be used to investigate the strata lying in differentgeographic directions from the axis of the bore hole being investigated.The device in Figure 6 may, therefore, be used in conjunction withorientation means known in the art, to determine transverse variationsin properties of strata penetrated by a bore hole, and to determine thegeographic direction of these variations.

A further modification of that form of the invention illustrated inFigure 4 is shown in Figures 7 and 8. Figure '8 is a horizontalsectional view of Figure 7 taken along the line 2121 as indicated. Inthese figures there are provided two ionization chambers 143 and 143'such as that illustrated in Figure 6. The disposition of ionizationchambers 143 and 143 is such that each occupies one-half of thecross-sectional area of the housing 137 and is slanted in the samemanner, as illustrated in Figure 6, but mutually opposite. It is to beunderstood that chamber 143 contains corresponding elements, as Well asa source of potential.

The output from chambers 143 and 143 .are fed to amplifiers 146 and146', respectively. Each amplifier output is conducted, in the mannerdescribed in connection with Figure 4, to independent recording circuitswhere separate records are made.

I The device of Figure 7 is sensitive to geographic orientation, and maybe oriented by methods familiar in the art.

Determination of the vertical dip of strata crossing a bore hole may bemadeby measuring the angle at Whichthe strata cross the bore hole. It isto be understood that this measurement has to be corrected by deviationsfrom vertical for the drill hole, which may be determined by methodswell known in the art. The magnitude of the dihedral angle between theplane of the strata and the plane perpendicular to the axis of the borehole may be determined by observing, on an enlarged .depth scale, theexact depths at which two corresponding transitions between adjacentstrata occur, and comparing these depths by subtracting them. It isunderstood that the instrument is so oriented that the above differenceis maximum, and that such orientation has been measured, to indicate thedirection of the dip. The angle of dip'is then 25ers where is the angleof dip A is the above described measured difference of depth 2 is thepenetration, or average depth of investigation of the radiationdetection process, out from the sides of the bore hole a W is thediameter of the bore hole Still another modification of the form of theinvention, as illustrated in Figure 4, is illustrated in Figure 9. Thismodification differs from those described immediately above in that theionization chamber and contained elements are given a conical form.There is employed in this modification a single ionization chamber.Generally speaking, the pointed portion of the chamber and containedelements point upwardly. In this modification, the same characterreferences are used to denote parts corresponding to those described inconnection with Figures 4 to 8.

This modification of an embodiment of the invention has thecharacteristic set forth in connection with Figures 4 to 8 inclusive,that it emphasizes, through its directional sensitivity,those fastneutrons which have been scattered from relatively heavy nuclei in thestrata (not hydrogen) at favorable angles of scattering. The directionalsensitivity of the device of Figure 9 is such that it preferentiallyobserves fast neutrons of maximum energy coming in on a cone having thesame apical angle as the apical angle of the conical elements of thechamber, a conical axis coincident with the axis of these elements, 141,142, 144, and an apex vertically situated between elements 142 and 144.The advantage of the device of Figure 9 is that it has no orientationsensitivity, therefore it gives a representative sampling of thematerial adjacent to the bore hole in all geographic directions from itsaxis.

The use of a favorable apical angle, together with a correct placementof the source 138 with respect to the apex of the ionization chamber143, gives better utilization of the neutrons, and greater sensitivity,because of the large number of neutrons scattered by nuclei of elementsin the strata at the more favorable scatting angle thus observed.

Obviously, the sensitivity of all the detectors of fast neutronsillustrated in Figures 4 to 9 will be limited by their small internalvolumes. This limitation may be overcome, while retaining the desireddirectional properties by repeating the elements of these radiationdetectors, connected electrically in parallel, and with all the elementsgeometrically parallel.

We claim:

' 1'. A method of specifically detecting hydrogenous material inunderground formations which comprises irradiatin g the formations witha flux of fast neutrons, detecting in the close vicinity of the point ofemission of fast neutrons only those neutrons of approximately maximumenergy which have been returned to the well from the formations at anobtuse scattering angle, measuring the intensity of the returnedneutrons relative to the intensity of neutrons similarly returned from aformation containing no hydrogen, and recording the measurement versusdepth.

2. A method of detecting neutrons incident upon a detector from selecteddirections which comprises measuring the frequency of occurrence ofrecoil processes occurring in the detector that are collinear with theincident neutrons from the selected directions, the said collinearrecoil processes being selected from all the recoil processes occurringin the detector in such a manner that the measured collinear processesare directed from the selected directions.

3. An apparatus for producing a neutron log of a well that comprises asource of fast neutrons, means for traversing the well with the sourceto effect bombardment of the formations with fast neutrons, meansdisposed 'adiaceut' said source and adapted for movement therewith fordetecting fast neutrons which have been diffused by the formations andreturned to the well, said detecting means comprising a shallowionization chamber, sensitive to direction of incident fast neutrons,means for recording signals resulting from said detection in,correlation with the depth at which detection occurred, means fortransmitting the signals from the detector to the recordingmeans.

An apparatus for producing a neutron log of a well that comprises asource of fast neutrons, means for traversing the well with the sourceto effect bombardment of the formations withfast neutrons, meansdisposed adjacent said source and adapted for movement therewithfordetecting fastneutrons which have been diffused by the formations andreturned to the well, said detecting means comprising a shallowionization chamber disposed at a substantial angle to the axis of thewell, 'means for recording, signals resulting from said detection incorrelation with the depth at which detection occurred, means fortransmitting the signals from the detector to the recording means.

5. An apparatus for producing a neutron log ofa well that comprises asource of fast neutrons, means for traversing the well with the sourceto effect bombardment of the formations with fast neutrons, meansdisposed adjacent said source and adapted for movement therewith fordetecting fast neutrons which have been diffused by the formations andreturned to the well, said detecting means comprising a pair ofshallowionization chambers dis posed at a substantial angle to the axisof the well but tilted oppositely, means for recording signals resultingfrom the separate detections in correlation with the depth at which thedetections occurred, means for transmitting the signals from thedetectors to the recording means.

'6. An apparatus for producing a neutron log of a well that comprises asource of fast neutrons, means for traversing the wellwith the source toeffect bombardment of the formations with fast neutrons, means disposedadacent said source and adapted for movement therewith for detectingfast neutrons which have been diffused by the formations and returned tothe well, said detecting means comprising a housing, an ionizable mediumin said housmg, substantially coaxial conical electrodes, having thesame apical angle, disposed in vertically spaced relallOnShlp in theionizable medium, means for recording signals resulting from saiddetection in correlation with the depth at which detection occurred,means for transmitting the signals from the detector to the recordingmeans.

7 method of detecting neutrons which comprises deriving small pulsesfrom all incident neutrons making a large angle with a specified planepassing through the detector, producing generally larger pulses fromneutrons incident upon the detector and directed general-1y parallel tothe specified plane, amplifying said small pulses and said largerpulses, suppressing the small amplified pulses, and measuring theunsuppressed pulses as an indicatlon of the number of neutrons havingdirections generally parallel to the said specified plane.

8. 'method of neutron well logging that comprises the steps'ofirradiating the formations penetrated by the well with fast neutrons,detecting only neutrons which have entered the formations and returnedto the well as a result of diffusion in the formations, said detectionresulting from a collection of electrons produced by recoil particlesresulting from collision between neutrons and atoms of the ionizablemedium within the detector, the number of electrons .collected followingeach collision being an index to the direction of the neutron whichproduces the process. Y.

9. In a neutron well logging system having a source of fast neutrons anda detector of fast neutrons disposed for movement therewith through aborehole, an apparatus for selectively measuring fast neutrons difiusedby the formations surrounding said borehole without having been diffusedby hydrogen comprising a shallow impulse ionization chamberdirectionally sensitive to the incident fast neutrons, discriminatingmeans for suppressing all 'but the larger pulses from said chamber, andmeans for measuring the rate of occurrence of the larger pulses.

10. In a neutron well logging system having a source of fast neutronsand a detector of fast neutrons disposed for movement therewith througha borehole, an apparatus for selectively measuring fast neutronsdiffused by the formations surrounding said borehole without having beendiflused by hydrogen comprising a shallow impulse ionization chamberdirectionally sensitive to the incident fast neutrons, discriminatingmeans for suppressing all but the larger pulses from said chamber, andmeans for measuring the rate of occurrence of the larger pulses, saidshallow ionization chamber comprising a confined ionizable gas having amajor portion of atoms of atomic weightless than 5, at least twoelectrodes of substantially the same shape and size disposed in contactwith said gas and separated by less than about one-seventh their leastdimension transverse to their separation, and means for applying voltagebetween said electrodes to collect electrons produced in said gas.

11. A shallow ionization chamber comprising a confined ionizable gashaving a major portion of atoms of atomic weight less than 5, at leasttwo electrodes of substantially the same shape and size disposedincontact with said gas and separated by less than about one-sevenththeir least dimension transverse to their separation, and means forapplying voltage between said electrodes to collect electrons producedin said gas.

References Cited in the file of this patent UNITED STATES PATENTS2,302,247 Neufeld Nov. 17, 1942 2,308,361 Fearon Jan. 12, 1943 2,316,361Piety Apr. 13, 1943 2,483,139 Herzog Sept. 27, 1949 2,575,769 Rajchmanet al. Nov. 20, 1951 2,599,352 Schneider June 3, 1952 2,710,925 McKayJune 14, 1955 2,735,953 Tirico Feb. 21, 1956 2,737,595 Scherbatskoy Mar.6, 1956

